Wideband Code Division Multiple Access (WCDMA) has been chosen as the radio technology for the paired bands of the UMTS. Consequently, WCDMA is the common radio technology standard for third-generation wide-area mobile communications. WCDMA has been designed for high-speed data services and, more particularly, Internet-based packet-data offering up to 2 Mbps in indoor environments and over 384 kbps for wide-area.
The WCDMA concept is based on a new channel structure for all layers built on technologies such as packet-data channels and service multiplexing. The new concept also includes pilot symbols and a time-slotted structure which has led to the provision of adaptive antenna arrays which direct antenna beams at users to provide maximum range and minimum interference. This is also crucial when implementing wideband technology where limited radio spectrum is available.
The uplink capacity of the proposed WCDMA systems can be enhanced by various techniques including multi-antenna reception and multi-user detection or interference cancellation. Techniques that increase the downlink capacity have not been developed with the same intensity. However, the capacity demand imposed by the projected data services (e.g. Internet) burdens more heavily the downlink channel. Hence, it is important to find techniques that improve the capacity of the downlink channel.
Bearing in mind the strict complexity requirements of terminals, and the characteristics of the downlink channel, the provision of multiple receive antennas is not a desired solution to the downlink capacity problem. Therefore, alternative solutions have been proposed suggesting that multiple antennas or transmit diversity at the base station will increase downlink capacity with only minor increase of complexity in terminal implementation.
The transmit diversity concept adopted for the FDD (Frequency Division Duplex) mode of third generation WCDMA system in the 3G standardization is currently being optimized for the case of two transmitting antennas at the base station.
In case of a so-called open-loop mode, a space-time block code is applied for the two transmitting antennas. The channel symbols are divided into two-element blocks which are transmitted from a first and second antenna, respectively, at successive time instants. These symbols are transmitted using the same spreading code. The receiver then uses a linear orthogonal processing based on the estimated channel coefficients to detect the transmitted symbols.
Alternatively, in case of a so-called closed-loop mode, a weight information is fed back from the terminals to the base station to approximate matched beamforming. FIG. 1 shows an example of such a closed-loop or feedback mode for a downlink transmission between a base station (BS) 10 and a mobile terminal or mobile station (MS) 20. In particular, the BS 10 comprises two antennas A1 and A2, and the MS 20 is arranged to estimate the channel on the basis of pilot channel signals used to probe the downlink channel and transmitted from the two antennas A1 and A2. Then, the MS 20 feeds back the discretized or quantized channel estimate to the BS 10. The antennas (or antenna elements) A1 and A2 are spaced sufficiently close to each other, so that the propagation delays between each of the antennas A1 and A2 and the MS 20 are approximately identical (within a fraction of a duration of a chip of the WCDMA spreading code). This is important in order to maintain downlink orthogonality in a single-path channel. Naturally, it is desired to develop a robust and low-delay feedback signaling concept.
Transmit diversity techniques provide a low-cost solution to increase downlink capacity in third generation systems. A number of different transmit diversity concepts have been developed. Both open and closed-loop concepts have significant merits in different environments and with different service assumptions.
In WCDMA, different modes have been suggested for the open-loop and closed-loop concepts which are optimized for two antennas. According to the open-loop Space-Time Transmit Diversity (STTD) mode, a two bit space-time code is used for transmission via the two antennas A1 and A2. Furthermore, closed-loop modes 1 and 2 (referred to as Transmission Antenna Array (TxAA) modes) have been specified, where feedback weights used for controlling power and/or phase of the transmission signals of the two antennas A1 and A2 are modified after a certain number of slots. The MS 20 estimates channel coefficients from common pilot signals (antenna or beam specific), wherein a simple dedicated channel estimate is derived from continuous common channel estimates. In particular, a quantized feedback is signaled to the BS 10 using the 1.5 kbps subchannel. In mode 1, the possible Tx feedback weights are selected from a QPSK constellation. In mode 2, the possible Tx feedback weights are selected from a 16-state constellation.
FIG. 2 shows a table indicating characteristic parameters of the above TxAA modes. In particular, NFB designates the number of feedback bits per time slot, NW the number of bits per feedback signaling word, Na the number of feedback bits for controlling an amplification or power at the antennas A1 and A2, and Np the number of feedback bits for controlling a phase difference between the antennas A1 and A2. As can be gathered from the table of FIG. 2, one bit is fed back per time slot in each of the feedback modes.
In the Tx AA mode 1, the feedback signaling word comprises two bits, and an update is performed after both feedback bits have been received, i.e. after two time slots. The feedback signaling word is only used for controlling the phase difference between the two antennas A1 and A2.
In the Tx AA mode 2, the bit length of the feedback signaling word is four, and an update is performed every four time slots. In particular, one bit of the feedback signaling word is used for controlling the amplification (power) at the antennas A1 and A2, and three bits are used for controlling their phase difference.
The required channel estimates are obtained e.g. from the common pilot channel signal transmitted with a known power from each antenna. In WCDMA systems, rather accurate estimates can be obtained by using the common channel pilots (CPICH) transmitted continuously from the two antennas A1 and A2. The feedback information can be transmitted in the Feedback Signaling Message (FSM) as a part of the FBI field of the uplink dedicated physical control channel (DPCCH).
It is to be noted that the STTD and TxAA modes may be implemented in an analogous manner in the beam domain. In the TxAA case, the MS 20 signals to the BS 10 whether to rotate the phase angle of channel signals transmitted from the antenna A2 by 180°. Then, the BS 10 transmits simultaneously from both antennas A1 and A2.
In the TxAA modes 1 and 2, the MS 20 transmits estimated and quantized channel parameters to the BS 10 which then weights the transmitted signals accordingly. Thus, a higher resolution than 180° (as provided by the STTD mode) can be achieved. The MS 20 selects the Tx weight (or Tx beam) from 4 or 16 different constellations, respectively.
As regards the table of FIG. 2, it is to be noted that an equal power is applied to the antennas A1 and A2 in the case where Na=0. Furthermore, the antennas A1 and A2 are uniquely defined by their respective pilot codes of the CCPCH (Common Control Physical Channel) of the UMTS. The derived amplitude and phase applied to the antennas A1 and A2 is called a weight and the set of weights is grouped into a weight vector. Specifically, the weight vector for the present case of two antennas is given by
      w    _    =      [                                                      PA              ⁢                                                          ⁢              1                                                                                                      PA                ⁢                                                                  ⁢                2                                      ·                          exp              ⁡                              (                                  jπΔφ                  /                  180                                )                                                          ]  wherein Δφ denotes the phase difference (phase weight) fed back to the BS 10. If more than two antennas, i.e. an antenna array, is used, the dimension of w becomes larger than two. In this case, a directional antenna may be achieved by using relative phases between antennas. The estimated phase of the feedback signal in the complex plane is then used for controlling the transmit direction.
Extensions to the above two-element concepts are considered in order to further increase the diversity gain. While many of these extensions are straightforward, most of them are applicable only in very low mobility environments, since the accuracy (or delay) of the feedback signaling is often compromised. In addition, it has to be considered that unnecessary transmissions might be performed if the channels of the corresponding antennas are in deep fades.
The basic application of 2-antenna or 2-beam Tx-diversity requires a doubling of the transmission power. This means that the diversity gain should exceed 3 dB in order to obtain a total diversity gain. From W. D. Jakes, “Microwave mobile communications”, IEEE Press, 1974, it is known that in a flat-fading channel with two, three, or four uncorrelated signal paths the gain (at 95% reliability level) compared to one-path reception is 8.5 dB, 12 dB, and 14.5 dB, respectively. Thus, the achievable Tx-diversity gain is approximately 5.5 dB when transmitting with two antennas instead of one, 1.5 dB with three antennas instead of two, and 1.5 dB with four antennas instead of three. This analysis was conducted for frequency non-selective channels, typically for micro-cells and indoor cases, assuming ideal feedback, channel estimation, combining and transmission. In frequency selective cases, the Rake receiver already exploits multipath (delay) diversity and hence the above gains of using multi-antenna or multi-beam diversity are reduced.
Moreover, the following problem associated with channel estimation has to be taken into account. If M antennas are used, the average transmission power of each antenna is reduced to P/M, wherein P denotes the total power required for the transmission. If m is large, the power per antenna is low and thus the performance of the channel estimation operation at the receiver will suffer due to the low SNR (Signal to Noise Ratio).